It is known that prehydrated bentonite can be added to salt water drilling fluids in order to increase their viscosity. The bentonite is prehydrated in fresh water, and the clay suspension thus obtained is added to the salty water. The drilling fluid thereby becomes more effective as a fluid for carrying out cuttings from the bore hole. It has also been proposed to include ferrochrome lignosulfonate in a prehydrated bentonite-containing drilling fluid additive mixture. The ferrochrome lignosulfonate is reported to stabilize the hydrated bentonite. It would be desirable to further improve the properties of a drilling fluid additive mixture comprising bentonite and ferrochrome lignosulfonate, particularly in order to increase the yield point of the drilling fluid. An increased yield point generally means that the fluid will carry more drill cuttings out of the drill hole. It would be particularly desirable to increase the yield point of the drilling fluid without increasing the plastic viscosity.